Vibrating structural gyroscopes (VSGs) have found use in a number of applications involving the detection of rotational rate and position, including attitude sensors and gyrocompasses. Rotational rate and/or position is typically determined by exciting a resonator along one or more drive axes to drive the resonator into an oscillation or vibration pattern and detecting a change in the output signal. The output signal may include a “quadrature” signal or component, herein defined as the component of a complex signal that is 90 degrees out of phase in the time domain with the in-phase component. This quadrature signal is generally considered an unwanted signal that can cause output errors.
Determination of quadrature signal from a rotation rate signal is known in the art. The quadrature signal may be obtained by demodulating the rotation rate signal out-of-phase with respect to the drive oscillation. Such determination of the quadrature signal by demodulation is presented in greater detail in a paper by Dr. D. D. Lynch, “Coriolis Vibratory Gyros,” presented at Symposium Gyro Technology, Stuttgart, Germany, 1998 (Lynch), and by U.S. Pat. No. 5,629,472 to Varnham et al. (Varnham).
Some VSGs are configured to drive the resonator assembly along a plurality of drive axes, with the drive axes being offset with respect to each other or, alternatively, being substantially coincident (i.e. defining substantially the same axis in three-dimensional space). The resonant frequency of a VSG resonator assembly will typically differ between respective drive axes. For example, a resonator assembly being driven along two in-phase drive axes typically has a different resonant frequency for the first drive axis than for the second drive axis; that is, each drive axis of the resonator assembly is said to have a unique resonant frequency. The resonator assembly may be designed and manufactured so that the resonant frequencies of the respective drive axes are close. Tuning processes may also be practiced to bring the respective resonant frequencies even closer. However, the resonant frequencies may never be perfectly tuned, particularly over a range of temperatures, because temperature can change the characteristics of the materials of the resonator assembly and cause a degradation of the tuning of the resonator assembly. This degradation and the resulting quadrature signal has been reported in the literature Lynch. Therefore, a quadrature signal can appear when both axes of a dual axis system are driven, resulting in an errant indication of rotation rate and/or rotational position.
A variety of options are available to the artisan to counter the effects of quadrature signals. Some methods focus on altering the vibration characteristics of the resonator. For example, Varnham discloses correction of the quadrature component by mechanically adjusting the resonator mass or mass distribution to tune the gyroscope. U.S. Pat. No. 4,951,508 to Loper et al. (Loper) discloses correcting the quadrature component by electrically adjusting the spring stiffness to tune the gyroscope. Other U.S. Patents and Published Patent Applications (e.g. U.S. Pat. Nos. 6,481,285 and 6,934,660 and U.S. Patent Application Publication No. 2007/0089510) also disclose this technique.
Likewise, U.S. Pat. No. 6,883,361 to Wyse (Wyse) discloses a method and apparatus whereby a DC voltage is introduced near a vibrating ring resonator to incite an oscillating force from the vibration, which coincidentally alters the stiffness of the vibrating element, which can be used to cancel quadrature component. Wyse discloses a “set and forget” system, with no dynamic adjustment for automatic or feedback control. Also, U.S. Pat. No. 6,675,630 to Challoner, et al. (Challoner) discloses a method and apparatus whereby a quadrature signal is applied as a DC bias voltage to affect a phase offset in the drive loop. The methods disclosed by Wyse and Challoner require at least one extra electrode in addition to the drive electrodes to accomplish the stiffening.
Other techniques focus on electronically compensating for the quadrature signal. For example, U.S. Pat. No. 7,120,548 to Malvern et al. (Malvern) discloses a technique whereby the quadrature signal is minimized by feeding a quadrature-correcting phased signal to a dedicated “torquing” element, causing a vibration that interacts with the driven oscillation pattern to drive the quadrature signal to a minimum, thereby actively correcting for the mistuning. U.S. Pat. No. 7,240,533 to Fell et al. (Fell I) presents a variation of this technique by including a phase corrector in the torque control loop that drives the quadrature torque energy directly into the sensed quadrature signal to correct for the effects of the quadrature component. Other examples where a quadrature signal is added to the torque signal to correct for quadrature signal include U.S. Pat. Nos. 7,188,522 and 7,216,525.
U.S. Pat. No. 7,231,823 to Schroder (Schroder) discloses a system wherein a “disturbance component” of the read signal is measured and a frequency offset is implemented as needed to match the disturbance. U.S. Pat. Nos. 7,249,488 and 7,337,665 disclose systems similar to Schroder.
Other systems, such as disclosed in “A Second Generation Silicon Ring Gyroscope” by C. Fell, I. Hopkins and K. Townsend (Fell II), utilize phase locked loops which control the oscillator so that there is either no phase difference or a known phase difference between the drive frequency and the oscillator frequency. Such systems are constantly being adjusted to lock in the phase relationship, and are therefore subject to phase jitter in the phase locked loop.
The disclosures of the above-mentioned patents and publications are hereby incorporated by reference herein in their entirety except for explicit definitions contained therein as follows: U.S. Pat. No. 5,629,472 (Varnham), U.S. Pat. No. 4,951,508 (Loper), U.S. Pat. No. 6,883,361 (Wyse), U.S. Pat. No. 6,675,630 (Challoner), U.S. Pat. No. 7,120,548 (Malvern), U.S. Pat. No. 7,240,533 (Fell I), U.S. Pat. No. 7,231,823 (Schroder), and paper by Dr. D. D. Lynch, “Coriolis Vibratory Gyros,” presented at Symposium Gyro Technology, Stuttgart, Germany, 1998 (Lynch).
U.S. Pat. No. 7,526,957 and U.S. Patent Application Publication No. 2007/0256495 to Watson (collectively “Watson”), both assigned to the assignee of the instant application and hereby incorporated by reference herein in their entirety except for explicit definitions contained therein, disclose drive axes that are rotationally skewed relative to the antinode axes of the vibration pattern when the VSG is rotationally at rest. The skewed axes enable the drive elements of a multiple drive axis system to affect a torquing function in addition to sustaining the oscillation pattern, thus eliminating the need for separate dedicated torque elements. Elimination of dedicated torque elements simplifies the resonator assembly and can provide a mirrored symmetry about a plurality of drive axes for more uniform propagation of vibration between the various nodes and antinodes of the system. Watson further discloses a method for minimizing or reducing the signals at the nodes of the oscillation pattern by changing the relative amplitudes of the drive signals along respective skewed drive axes, thus shifting the position of the node on the resonator.
The above disclosed techniques and systems that focus on electronically compensating for the quadrature signal do not improve the tuning of the gyroscope. That is, each of the disclosures imposes a force (e.g., the separate torque elements of Fell I or the differing amplitudes of Watson) or simply establishes the error as a known quantity (e.g., the phase-lock system of Fell II). None of these systems or techniques improve the tuning of the resonator assembly. The quadrature signal itself is indicative that energy introduced in the plurality of drive axes is dissipated in destructive interference. Systems that introduce additional forces to accomplish the compensation introduce still more energy that is also dissipated in destructive interference. Such dissipated energy is in many instances transferred to the structure supporting the resonator assembly, and can be reflected back to the resonator, causing additional signals of arbitrary phase that results in further biasing error in the rotational rate signal.
A vibrating structural gyroscope system that electronically improves the tuning of the VSG with respect to the inherent tuning error represented by the quadrature signal, rather than merely attempting to compensate for the inherent tuning error of the VSG, would be welcome.